The present invention relates to an eddy current coupling and more particularly to an eddy current coupling having an interdigitated pole assembly wherein the numerical value of the total face surface area of each of the plurality of pole members is 2.2 to 3 times larger than the numerical value of the crossectional area of the core portion of the pole assembly to reduce the flux density at the pole face to enable the coupling to maximize torque output at low slip speeds.
Eddy current couplings are known in the art. In such known eddy current couplings generally it has been thought that it was desirable to maintain a constant flux density throughout the pole assembly. Hence, the pole teeth on many interdigitated pole members were tapered toward the face surface area to provide for a constant flux flow path from the core of the coil to the face surface area taking into account inter-pole leakage. An example of such a construction is disclosed in the Winther et al. patent entitled "Toroidal Magnet Field for Dynamoelectric Machines," U.S. Pat. No. 2,470,596, and in the Jaeschke patent entitled "Magnetic Pole," U.S. Pat. No. 2,606,948. In such couplings the torque vs. slip curve (as shown at 70 in FIG. 5) rises slowly at low slip and has a relatively flat configuration wherein the output torque is maximized during high slip conditions. The flattened portion of the curve where torque is maximized is desirable in many industrial applications wherein it is desired to operate the coupling over a wide range of relatively high slip speeds with a substantially constant torque output.
In cases where high torque at low slip has been required, a salient pole coupling has been utilized. A salient pole coupling provides high torque at low slip. The torque curve then droops under high slip conditions due to the fact that at high slip the flux emanating from the pole assembly is overcome and cancelled by the flux emanating from the inductor drum. Such a torque transmitting characteristic is desirable in certain industrial situations, such as pump drives or fans wherein the drive is mainly run at low slip conditions wherein maximum torque is desired. However, salient pole couplings require a separate coil on each pole member. This substantially increases the cost of salient pole couplings over interdigitated couplings. Thus, it is desirable to provide an interdigitated coupling with the torque characteristics of a salient pole coupling in certain situations. If such is provided, the interdigitated pole coupling will cost considerably less than the salient pole coupling hence a considerable cost savings will occur.